The present invention relates to a multiband filter circuit that has a plurality of passbands and to a high frequency transmitter that employs the circuit.
The present specification supposes a high-frequency band system for digital wireless communications and so on, and therefore, filter characteristics are hereinafter each expressed in a 50-Ω system S-parameters graph. With regard to the filter characteristics, an S11-parameter represents a reflection coefficient, and an S21-parameter represents a transmission coefficient. Moreover, all the graphs of the S11-parameter and the S21-parameter show theoretical calculation results obtained by a general circuit simulator (GENESYS7.0 of EAGLEWARE Corp.) available on the market.
First of all, the fundamental BPF (bandpass filter) circuit will be described before proceeding to the explanation of a multiband filter circuit. FIG. 11 shows one example of the structure of a most fundamental prior art BPF.
FIG. 11 shows a well-known circuit structure that is practically often used in a high-frequency band and serves as a resonator-coupled type bandpass filter circuit constituted mainly of electric field coupling. This bandpass filter circuit is the circuit that is disclosed in, for example, “Design and Applications of filter circuits for communications” (written by Yoshihiro Konishi, published by Sougou-Denshi Shuppan, 1st Edition, p.26, FIG. 2.27(b)) and has a number n of stages of n=2.5 commencing from the series capacitance located between the stages. In FIG. 11, there are shown resonator sections 1 and 2. One example of the characteristic of this bandpass filter circuit is shown in FIG. 14, which includes only one passband. The circuit constants when this result was obtained were C1g=C2g=5.451 pF, C3g=C5g=1.343 pF, C4g=0.679 pF, and L1g=L2g=2.267 nH.
Although there is shown the case of the circuit of the minimum construction that has only two resonator sections 1 and 2, it is well known that the number of stages can easily be increased or decreased in such a bandpass filter circuit. Although the present specification provided below mentions only circuit diagrams and simulation results in the case where the number of resonators is two for the sake of simplicity of explanation, neither the prior art nor the present invention described in the present specification is limited by the number of resonators.
Although the case of an ideal equivalent circuit constructed of only lumped-constant elements (L and C) is shown in FIG. 11, it is well known that this circuit can also be provided by a distributed constant circuit. For example, FIG. 12 shows an example in which only the resonator sections 1 and 2 in FIG. 11 are replaced by λ/4 shortcircuit resonators TL1h and TL2h of distributed constant transmission lines. FIG. 13 shows an example in which the capacitance elements C1h, C2h and C3h in FIG. 12 are replaced by gap capacitances 27, 28 and 29 between the distributed constant transmission lines and the λ/4 shortcircuit line resonators TL1h and TL2h are replaced by λ2 open-circuited line resonators 25 and 26. In FIG. 13, there are shown a dielectric substrate 21 of ceramic, glass epoxy or the like, a ground pattern 22 on the lower surface of the substrate, and metal wiring patterns 23 and 24 formed on the upper surface of the substrate. The patterns 23 and 24 constitute a high-frequency transmission line called the microstrip line in the case of FIG. 13.
In the present specification provided below, circuit diagrams and simulation results in the distributed constant transmission line form as in FIGS. 12 and 13 are not shown for the sake of simplicity of explanation. However, neither the prior art nor the present invention described in the present specification is limited to the lumped constant form as in FIG. 11.
As is apparent from FIGS. 11 through 13, the resonator-coupled type bandpass filter circuit is the technology that has a number of derivative forms and is widely put to practical use. It is impossible to describe all these derivative forms in the present specification. The essence of the present invention described later is shown in FIGS. 7 through 9, and the structure of the bandpass filter circuit is not limited only to FIGS. 11 through FIG. 13.
The present specification is focused on a filter circuit that has a plurality of passbands, or a so-called multiband filter circuit.
There is a prior art multiband filter circuit shown in FIGS. 15 and 16 (refer to, for example, JP 2000-124705 A (see FIGS. 4, 13 and 26).
FIG. 15 shows a figure of the principle of a first prior art multiband filter circuit. This multiband filter circuit shown in FIG. 15 has a structure in which two bandpass filter circuits 31 and 32 of different center frequencies are connected parallel to each other. Impedance matching cannot be achieved if the circuits 31 and 32 are simply connected parallel to each other and the filter waveform disadvantageously collapses. Therefore, matching is achieved by four phase shifters 33, 34, 35 and 36. Due to the four phase shifters 33, 34, 35 and 36, the multiband filter circuit of FIG. 15 generally tends to have a large size.
Moreover, FIG. 16 shows a figure of the principle of a second prior art multiband filter circuit. This multiband filter circuit shown in FIG. 16 has a structure in which two bandpass filter circuits BPF1i and BPF2i of different center frequencies are connected parallel to each other. It is to be noted that this circuit differs from the multiband filter circuit of FIG. 15 in that the individual bandpass filter circuits 31 and 32 have different structures and two (35, 36) of the phase shifters are removed. However, the other two (33, 34) of the phase shifters remain unable to be removed, and this is a factor for hindering the size reduction of the circuit.
The prior art multiband filter circuits as described above have several problems.
A first problem is that the number of passbands to be provided is able to be increased up to two but not able to be increased to three or more. For example, according to the recent portable telephones, there has been demanded a triple-band type terminal that utilizes three or more frequency bands such as the initial PDC system (800-MHz band) and the subsequently expanded PDC system (1.5-GHz band) and W-CDMA system (2-GHz band). It is difficult for the prior art multiband filter circuits to cope with such a high frequency transmitter.
A second problem is that the degree of steepness of the filter characteristic is low as described later. In the case of a single unit bandpass filter circuit as shown in FIG. 11, there is known a designing method for increasing the degree of steepness by forming a transmission zero in the vicinity of the passband as a derivation form. However, in the case of the multiband filter circuit of which the structure is more complicated, it is the current situation that no design theory for freely forming a transmission zero has been established. In the case of the second prior art multiband filter circuit (FIG. 16), it is disclosed that a total of four transmission zeros can be formed in JP 2000-124705 A. However, as is apparent from this document of JP 2000-124705 A, a transmission zero is formed only in a frequency band comparatively remote from the passband, and the degree of steepness is consequently not so improved (particularly in the vicinity of the higher frequency side of the passband).
A third problem is that it is difficult to reduce the size of the circuit. The problem of the large size of the first prior art shown in FIG. 15 is pointed out in the document of JP 2000-124705 A. Moreover, according to the second prior art shown in FIG. 16, the two transmission lines TL31i and TL41i for phase adjustment cannot be removed as already pointed out in the present specification, and therefore, it is also difficult to reduce the size of the circuit in the low frequency band in which the wavelength λ is long.